#include <iostream>
#include <cmath>

using namespace std;

#define s(a,b,c) (a+b+c)/2
#define a(a,b,c) sqrt(s(a,b,c)*( s(a,b,c)-a)*(s(a,b,c)-b)*(s(a,b,c)-c))

int main ( )
{
    double a, b, c,s;

   cout<<"enter the edge length of the triangle"<<endl;
   cin>>a>>b>>c;
   s=a(a,b,c);
   cout<<s<<endl;

   cin>>a;
   return 0;
}